1. Field of the Invention
This invention relates to a high-breakdown-voltage semiconductor element.
2. Description of the Related Art
A technique of providing a guard ring around an end pn-junction formed in an element region to be supplied with a high voltage is known as a method of increasing the breakdown voltage of a semiconductor element. The guard ring reduces the degree of electric field concentration at a tip portion of a depletion layer extending from the pn-junction, thereby increasing the breakdown voltage of the element. The effect of reducing the degree of the electric field concentration is obtained by optimally designing the depth of diffusion of the guard ring layer, the distance between the guard ring layer and a terminal region, and the distance between guard ring layers (if a plurality of guard ring layers are employed).
FIG. 16 shows an end portion of a high-breakdown-voltage diode having such a guard ring structure. A high-concentration p-type layer 2, which is to be an anode region, is formed in the upper surface of a high-resistance n-type silicon layer 1 (substrate). Further, a high-concentration n-type layer 3 is formed in the upper surface of the layer 1 separated from the layer 3 by a predetermined distance, for stopping the extension of a depletion layer. The layer 3 is provided with an electrode 5 for supplying a voltage identical to that of a cathode. High-concentration p-type layers 21 and 22 serving as guard rings are formed in the surface between the layers 2 and 3. An electrode 7 is formed on the lower surface of the layer 1 with a high-concentration n-type layer 6 interposed therebetween.
In such a structure, it is difficult to determine the distance between a guard ring layer and a terminal region, and the distance between guard ring layers in the case of employing a plurality of guard ring layers.
As described above, the conventional high-breakdown-voltage semiconductor element of a guard ring structure is hard to design.